U.S. patent number 4,910,438 [Application Number 06/810,399] was granted by the patent office on 1990-03-20 for wide band, high efficiency simmer power supply for a laser flashlamp.
This patent grant is currently assigned to Hughes Aircraft Company. Invention is credited to Robert P. Farnsworth.
United States Patent |
4,910,438 |
Farnsworth |
March 20, 1990 |
Wide band, high efficiency simmer power supply for a laser
flashlamp
Abstract
Methods and apparatus are disclosed for supplying and
controlling simmer current to a flashlamp 28. A direct
current-to-direct current converter including a transformer 12,
full-wave bridge rectifier 24, and first and second switching means
20 and 22 produces an output which is always higher than the
maximum required by the flashlamp 28 at its lowest simmer current.
The converter (12, 20, 22, 24) is coupled to flashlamp 28 through
an inductor 26. The current I(s) through a pair of inductors 26, 30
is sensed by a lamp current sensing circuit 34 which turns the
converter on or off, respectively, when a preselected minimum or
maximum value is reached. The converter transformer 12 is
maintained in its operating range by the switching of two devices
such as power FET's 20, 22 so that one or the other of the two
halves of the primary winding 14 is used alternately. The initial
ionization of flashlamp 28 is provided by simmer trigger circuit
38. A transformer primary current sensing circuit 40 controls the
toggling action which alternates the operation of switching means
20 and 22. The advantages of this method for supplying and
controlling flashlamp simmer current include fast current response,
extended flashlamp life, simplicity of design, high efficiency, and
the avoidance of saturation and synchronization problems.
Inventors: |
Farnsworth; Robert P. (Los
Angeles, CA) |
Assignee: |
Hughes Aircraft Company (Los
Angeles, CA)
|
Family
ID: |
25203770 |
Appl.
No.: |
06/810,399 |
Filed: |
December 17, 1985 |
Current U.S.
Class: |
315/241P;
315/174; 315/241S; 315/307 |
Current CPC
Class: |
H05B
41/34 (20130101) |
Current International
Class: |
H05B
41/30 (20060101); H05B 41/34 (20060101); H05B
041/16 () |
Field of
Search: |
;315/113,174,178,241R,243,246,232,224,208,225,237,307,324,241P,241S |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Groody; James J.
Assistant Examiner: Powell; Mark R.
Attorney, Agent or Firm: Streeter; William J. Denson-Low;
Wanda K.
Government Interests
BACKGROUND OF THE INVENTION.
U.S. Government Rights.
The United States Government has certain rights in this invention,
which was developed under Contract No. DAAK10-81-C-0093 awarded by
the United States Army.
Claims
What is claimed is:
1. A method of continuously maintaining a simmer current in a
gaseous discharge device in order to insure that said gaseous
discharge device is always held in a state of full power readiness
including the steps of:
generating a gaseous discharge device input voltage which is always
greater than a maximum gaseous discharge device voltage which
corresponds to a lowest gaseous discharge device simmer current,
the simmer current flowing through a first inductance and through a
second inductance, a first terminal of each of the first and second
inductances being coupled together, a second terminal of the second
inductance being coupled to the gaseous discharge device;
coupling the generated input voltage to the first terminals of the
first and second inductances;
sensing a magnitude of the simmer current at a second terminal of
the first inductance; and
adjusting the gaseous discharge device input voltage in response to
the magnitude of the simmer current so that the simmer current
cycles continuously within a preselected range of values.
2. A method of providing a simmer current to a flashlamp including
the steps of:
applying a simmer current to the flashlamp through a first
inductance and through a second inductance, a first terminal of
each of the first and the second inductances being serially coupled
together, a second terminal of the second inductance being coupled
to the gaseous discharge device;
sensing the simmer current within the flashlamp with a flashlamp
current sensing circuit coupled to the flashlamp at a second
terminal of the first inductance;
comparing the sensed simmer current to an upper current limit and
to a lower current limit stored in a flashlamp current sensing
circuit which is coupled to a direct current-to-direct current
converter transformer, an output of the transformer being coupled
to the first terminals of the first and the second inductances;
increasing the simmer current if the simmer current is equal to the
lower limit by
energizing the direct current-to-direct current converter
transformer, the converter transformer providing an increase in
output voltage which, in turn, increases the simmer current;
and
decreasing the simmer current if the simmer current is equal to the
upper limit by
de-energizing the converter transformer, which, in turn, decreases
the simmer current in order to maintain the simmer current within a
predetermined range.
3. Apparatus for providing and controlling simmer current to a
gaseous discharge device comprising:
a first inductance and a second inductance serially coupled
together at a first terminal of each of said first and said second
inductances, said second inductance further being coupled to a
gaseous discharge device such that said discharge receives a simmer
current though said first and said second inductances;
a current sensing means for sensing the simmer current in said
gaseous discharge device, said current sensing means having an
input coupled to a second terminal of said first inductance;
power means having an output coupled to said first terminals of
said first and second inductances for supplying said simmer current
to said gaseous discharge device;
first and second switching means coupled to said power means for
selectively energizing said power means in order to provide said
simmer current within a preselected current range; and
control means having a first input coupled to an output of said
current sensing means and an output coupled to said first and
second switching means for regulating the alternation of operation
of said first and second switching means.
4. Apparatus as claimed in claim 3 in which:
said power means is a direct current-to-direct current converter
including a transformer which further includes a primary winding
and a secondary winding, and which is further coupled to a
full-wave bridge rectifier.
5. Apparatus as claimed in claim 3 in which:
said first and second switching means are field effect
transistors.
6. Apparatus as claimed in claim 3 in which:
said gaseous discharge device is a flashlamp for a laser.
7. Apparatus as claimed in claim 4 in which:
said control means comprises a second input coupled to an output of
a transformer primary current sensing circuit.
8. Apparatus as claimed in claim 4 in which:
said direct current-to-direct current converter transformer has a
ratio of primary windings to secondary windings which produces a
higher output voltage than a maximum voltage required by said
gaseous discharge device at a preselected low limit of said simmer
current.
9. Apparatus as claimed in claim 4 in which:
said first and said second switching means are each coupled to said
primary winding of said transformer, and wherein:
said current sensing means de-energizes said primary winding of
said direct current-to-direct current converter transformer by
de-activating at least one of said first and second switching means
if said gaseous discharge device current exceeds a preselected
maximum current.
10. Apparatus as claimed in claim 4 in which:
said first and said second switching means are each coupled to said
primary winding of said transformer, and wherein:
said current sensing means energizes said primary winding of said
direct current-to-direct current converter transformer by
activating one of said first and second switching means in an
alternating sequence if said gaseous discharge device current falls
below a preselected minimum current.
11. A simmer power supply for a flashlamp comprising:
a direct current-to-direct current converter transformer which
includes:
a primary winding,
a secondary winding,
a center input tap,
a first FET switching device coupled to said primary winding,
a second FET switching device coupled to said primary winding,
and
a full-wave bridge rectifier;
said direct current-to-direct current converter transformer being
coupled to said flashlamp through a flashlamp coupling
inductor;
a flashlamp current sensing circuit coupled to said direct
current-to-direct current converter transformer through a lamp
current sensing circuit coupling inductor;
a transformer primary current sensing circuit coupled to said
primary winding of said direct current-to-direct current converter
transformer;
a transformer primary current sensing circuit coupling resistor
connected to said transformer primary current sensing circuit and
to ground; and
a simmer trigger circuit connected to said flashlamp through a
simmer trigger circuit coupling resistor.
12. Apparatus for maintaining a simmer current in a gaseous
discharge device, comprising:
means for generating a gaseous discharge device input voltage
having a magnitude which is greater than a maximum gaseous
discharge device voltage which corresponds to a lowest gaseous
discharge device simmer current, the simmer current flowing through
a first inductance and through a second inductance, a first
terminal of each of the first and second inductances being coupled
together and a second terminal of the second inductance being
coupled to the gaseous discharge device;
means for coupling the generated input voltage to the first
terminals of the first and second inductances;
means for sensing a magnitude of the simmer current at a second
terminal of the first inductance; and
means for regulating the magnitude of the gaseous discharge device
input voltage in response to the sensed magnitude of the simmer
current such that the magnitude of the simmer current is maintained
within a preselected range of values.
13. Apparatus as defined in claim 12 wherein said means for
generating comprises:
a DC-to-DC converter transformer having a primary winding and a
secondary winding, the secondary winding having a full-wave bridge
rectifier coupled thereacross, the first terminals of each of the
first and second inductances being coupled together through the
full-wave bridge rectifier.
14. Apparatus as defined in claim 13 wherein said transformer
primary winding has a first and a second FET device coupled to a
first and a second end, respectively, of the primary winding and
wherein the primary winding further has a tap coupled to a source
of DC voltage.
15. Apparatus as defined in claim 14 wherein said regulating means
comprises means coupled to a gate of each of the FET devices for
alternately energizing and deenergizing the FET devices in response
to the sensed magnitude of the simmer current.
16. Apparatus as defined in claim 15 wherein said regulating means
further comprises primary current sense means having an input
coupled to the primary winding for sensing a magnitude of the
primary winding current which exceeds a predetermined maximum
magnitude, the primary current sense means further having an output
coupled to the gate of each of the FET devices for deenergizing
both of the FET devices when the predetermined maximum magnitude is
sensed.
17. Apparatus as defined in claim 12 wherein each of the first and
second inductances has an inductance value of approximately 125
microhenries.
Description
FIELD OF THE INVENTION.
The present invention relates to laser pumping power supplies, and,
more particularly, to means for providing and controlling simmer
current to a gaseous discharge device such as a flashlamp
associated with laser optical pumping.
DESCRIPTION OF THE TECHNOLOGY.
Laser gain media receive excitation energy from a variety of
flashtube or flashlamp devices which initiates the process of light
amplification. A flashlamp is capable of producing light from
energy stored in a continuously ready supply. This light is
generally composed of blackbody radiation which includes light
energy having frequencies that coincide with the frequencies of
transition between the ground states and the pumping bands of the
laser medium. It is this portion of flashlamp output which
stimulates the emission of radiation by inducing a population
inversion within the gain medium.
It is advantageous to maintain a simmer current through the
flashlamp at a comparatively low power level compared to that
needed for full firing of the flashlamp in order to improve output
energy stability from the laser and to increase flashlamp life.
Enhancing flashlamp life is a critically important design
objective, since systems which incorporate devices similar to the
present invention are expected to function without fail over a
million to ten million times. The maintenance of a steady simmer
current enables a pulsed arc to be formed in the flashlamp in an
almost instantaneous response to a pulsed power source. As a
result, the operational life of the flashlamp is increased because
of reduced physical shock from the high-energy pumping pulses,
reduced devitrification of the flashlamp envelope, and minimization
of electrode sputtering and evaporation.
The importance of keeping a flashlamp in a simmer condition so that
it is ready to fire at the time the main pumping energy is applied
to the gain medium is well known to those persons ordinarily
skilled in the art. Several earlier devices pertain to this general
area of technological enterprise. Among these devices, a few
inventions have attempted to solve the problem of placing a
flashlamp in a simmer condition.
In U.S. Pat. No. 4,398,129, Logan describes a flashlamp drive
circuit using an unsaturated transistor as a current mode switch to
periodically subject a partially ionized gaseous laser excitation
flashlamp to a stable, rectangular pulse of current from an
incomplete discharge of an energy storage capacitor. The pulse
interval is set by a monostable multivibrator, and the pulse
amplitude is controlled by a feedback signal provided by a tap on
an emitter resistor. The circuit drives a flashlamp to provide a
square wave current flashlamp discharge. After the lamp has been
placed in a partially ionized state by an ignition voltage pulse
from an external source, a transistor in the circuit, held below
saturation in an active, common-emitter configuration, cyclically
switches the amplitude of current between two modes--a simmer
condition maintaining partial ionization and a total ionization
pumping condition. The Logan patent is primarily concerned with the
utilization of an active device in series with a flashlamp to
control flashlamp current and the laser pumping process. The
circuitry disclosed in this patent does not teach methods or
apparatus for actively controlling the simmer current. Logan
explains that the simmer condition developed by his device fails if
any load is imposed which severly limits the bandwidth of the
simmer state segment of his lamp driver circuit. The Logan
invention appears to be more concerned with using an active device
to control the shape, duration, and timing of the laser output
pulse during full, high current discharge than with the regulation
of simmer current.
In U.S. Pat. No. 4,035,691, Altman et al. disclose a pulsed laser
excitation source with light energy output which is principally in
the 3600 to 4300 Angstrom spectral region. An envelope of material
that is substantially transparent to radiation within the desired
spectral region contains an amount of xenon gas which produces at
least one atmosphere of pressure in its unheated state. The
envelope also contains an amount of mercury which develops a vapor
pressure of not less than one atmosphere when the envelope is
heated to its operative temperature. When spaced electrodes within
this sealed envelope are energized by an external source of pulsed
electrical power, an electrical arc or discharge is generated. In
one embodiment of the Altman invention, the direct current simmer
current in the light source is maintained within the range of 15 to
25 amperes in order to facilitate the breakdown of the xenon gas
and thus insure pulsed arcing across the spaced electrodes in
virtually instantaneous response to the pulsed power source. A
preferred embodiment of this invention may include a suitable
direct current power source as well to expedite and facilitate the
breakdown of the xenon gas.
U.S. Pat. No. 4,005,333--Nichols describes apparatus for increasing
the output efficiency of an optically coupled Nd:YAG (Neodymium:
Yttrium-Aluminum-Garnet) laser by turning on a flashlamp slightly
at a predetermined interval of time before the laser is fully
energized. This invention utilizes a pseudosimmer concept in which
a small quantity of pump energy is applied to the flashlamp just
before the main discharge of energy is released to stimulate the
laser gain medium. Nichols asserts that this method of triggering
results in a larger ionic path diameter at the time of the main
energy release to the flashlamp. Current flow from the main energy
supply through the flashlamp is controlled by a silicon controlled
rectifier. Nichols' simmer pulse network is characterized by a
capacitor that stores energy, which, when allowed to flow, is
conducted serially through a saturable inductor, an isolating
diode, and the flashlamp.
Dere et al. disclose an optical cavity for a flashlamp pumped dye
laser in U.S. Pat. No. 3,967,212 which includes a pumping cavity, a
birefringent filter, and a plurality of frequency doubling
crystals. Within the pumping cavity, the flashlamp is cooled by
forced air convection and is operated with a direct current simmer
current. The inventors of this device claim that the application of
the direct current arc or simmer current to the flashlamp increases
the operational lifetime of the flashlamp. Dere et al. also assert
that their technique reduces the physical shock of high energy
pumping pulses endured by the flashlamp, diminishes envelope
devitrification, and minimizes electrode sputtering and
evaporation. Although the Dere patent mentions the use of a simmer
power supply to increase the operating life of a flashlamp, the
inventors do not teach or suggest apparatus or methods for
producing simmer current as claimed in the present invention.
U.S. Pat. No. 4,267,497 contains a description of a power supply
for a laser flashtube or lamp such as a continuous wave arc lamp.
Burbeck et al. describe a high frequency switch for providing a
pulse train output from a direct current supply. Before a pulse
train output is imposed across the flashtube or lamp, the pulse
width of the signal is modulated and a portion of the high
frequency ripple in the signal is removed by a filter. The
modulated pulse train output may be raised to a direct current
level in order to supply simmer current to the flashtube or lamp.
The high frequency switch may consist of at least one transistor or
thyristor. The filter may be an arrangement of inductors and
capacitors.
This Burbeck invention comprises a switching type regulator which
supplies a controlled current to a flashlamp. This prior device
employs a sensing technique which is more concerned with the
current through the lamp and the generation of a constant low
ripple lamp current, as opposed to sensing the voltage across the
flashlamp. The level of simmer current which is needed to maintain
conduction in the Burbeck flashlamp is relatively high since the
simmer current is generated from a relatively low impedance
source.
None of the inventions described above responds quickly to changes
in the flashlamp voltage, regulates simmer current using a
d.c.-to-d.c. converter in a single control loop, maintains the
flashlamp simmer current between preselected, desirable upper and
lower levels, and avoids saturation and synchronization
difficulties encountered by many prior devices. None of these prior
methods or apparatus provides an effective and comprehensive
solution which addresses all of the complex aspects of this simmer
current problem. Such a solution to this problem would satisfy a
long felt need experienced by the laser industry for over two
decades. A truly practical and reliable means for producing an
efficacious simmer current that would extend laser flashlamp life
substantially would constitute a major advancement in the
optoelectronics field. Manufacturers of laser devices could employ
such an innovative design to produce lasers which would be capable
of instantaneous firing at full power on demand. Such an invention
would ideally be suited to operate in cooperation with a wide
variety of coherent radiation systems and would perform
consistently and reliably over a wide range of operating conditions
and system applications.
SUMMARY OF THE INVENTION
The present invention provides an efficacious, practical,
cost-effective, compact, and straightforward solution to the
problem of generating, regulating, and controlling a suitable
simmer current which will enhance the performance and operating
lifetime of laser flashlamps and similar gas discharge lamps. This
invention utilizes a d.c.-to-d.c. converter to generate an output
voltage which is higher than the maximum voltage required by the
flashlamp at the lowest simmer current. This converter is connected
to the flashlamp by an inductor. The current through this inductor
is sensed by an actuator circuit which turns the d.c.-to-d.c.
converter on when a minimum inductor current is sensed, and turns
the converter off when a maximum current flows through the
inductor. The simmer current is automatically, rapidly, and
reliably maintained between preselected limits. This automatic
regulation is accomplished without producing deleterious current
spikes which are generated by transformer saturation. By
maintaining a continuous simmer condition, the Farnsworth flashlamp
simmer power supply insures that the flashlamp is constantly in a
state of full power readiness. The lamp remains on for extended
periods because the simmer condition preserves a minute filament of
conducting ions between its internal electrodes. Even after the
flashlamp operating characteristics have been degraded over time,
the simmer circuit still operates, since it is capable of constant
and substantially instantaneous adjustment to the changing
flashlamp current. This enormous dynamic range and stability of
operation allows this innovative device to be used with a broad
variety of laser systems under greatly disparate environmental
conditions. The present invention controls simmer current without
the saturation and synchronization problems which plague prior
devices that incorporate interacting control loop designs. While
previous contrivances have attempted to provide simmer current by
sensing average current through a flashtube and responding slowly
to correct an imbalance, the present invention supplies a remedy
which is instantaneous. The present invention also constitutes an
achievement in simplification of previously complex electronic
design. The switching components used in the invention claimed
below represent an evolution beyond the multiple switching loops
and all their concomitant problems which have been utilized in the
past. The Farnsworth simmer current invention further includes a
circuit design which is substantially immune from noise and Joule
heating losses which greatly debilitate flashlamp performance.
It is, therefore, an object of the present invention to
substantially increase the operating life of a flashlamp or gas
discharge device by providing an automatic, precisely controlled,
regulated simmer current which maintains the flashlamp or gas
discharge device in a continuously but only slightly active or
simmer condition in order to provide instantaneous, full power
capacity on demand. By precluding the necessity of repeatedly
having to reinitiate the full discharge arc, the continuous low
power arc totally eliminates the need to repetitively start the
flashlamp from a completely cold condition.
It is a further object of the invention to provide a simmer current
system which can respond to changes in flashlamp current in
extremely short time periods.
It is also an object of this invention to ensure that the flashlamp
current never exceeds the preselected upper limit nor drops below
the preselected lower limit, so long as the lamp requirements are
between zero and the maximum voltage of the supply.
Another object of the present invention is to control the simmer
current by employing a regulating direct current-to-direct current
converter in a circuit arrangement that comprises a single control
loop, so that synchronization and other difficulties associated
with previous designs are avoided.
Still another object of the invention is to avoid transient drops
in flashlamp current in the event that the d.c.-to-d.c. converter
transformer should experience saturation.
It is a further object of the invention to provide simmer power to
a flashlamp more efficiently than in previous designs by minimizing
resistive heat losses in control circuits.
Yet another object of this Farnsworth invention is to supply well
regulated flashlamp simmer current that is relatively immune from
the deleterious consequences of noise or voltage fluctuation within
the lamp.
An appreciation of other aims and objects of the present invention
and a more complete and comprehensive understanding of this
invention may be achieved by studying the following description of
a preferred embodiment and by referring to the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram that depicts essential elements of
the present invention including the flashlamp simmer current supply
and control means.
FIG. 2 illustrates regulation curves for the Farnsworth simmer
power supply for driving fixed resistor loads.
FIGS. 3(a), (b), (c), and (d) depict traces which reveal the
details of one regulation cycle of the present invention over forty
microseconds.
FIG. 4 is a graph of the voltage V(1) across a typical flashlamp
versus the current through the flashlamp I(1) showing the various
limiting values for the simmer current, simmer voltage, and supply
voltage.
FIG. 5 is a dual oscilloscope trace showing a typical example of
FET drain voltage (top) and drain current (bottom).
FIG. 6 is another oscilloscope trace revealing the cyclical rise
and fall of the flashlamp current which results from the simmer
power condition created by the present invention.
FIG. 7 is a schematic diagram showing the transformer primary
current sensing circuit and the lamp current sensing circuit.
DESCRIPTION OF A PREFERRED EMBODIMENT
FIG. 1 depicts a diagram of the simmer current power supply 10 in
abbreviated schematic form. A direct current-to-direct current
converter transformer 12 includes a primary winding 14, a secondary
winding 16, and a direct current input center tap 18. In this
preferred embodiment, the primary to secondary winding ratio is
thirty to one and the voltage introduced at input tap 18 is
twenty-eight volts. The primary winding 14 is coupled to first
switching means 20 and second switching means 22 illustrated
schematically as on/off switches. In the preferred embodiment,
first and second switching means are field effect transistors
(FETs). The output of secondary 16 is connected in parallel to full
wave rectifying bridge 24 which includes four diodes 24a,b,c,d. The
rectifying bridge 24 is coupled in series to the anode of the
flashlamp 28 through flashlamp coupling inductor 26. The best mode
of the present invention employs a 125 microhenry coil for inductor
26. The current through inductor 26 is shown as I(s). The cathode
or negative terminal of the flashlamp is connected to ground.
A lamp current sensing circuit 34 is connected to rectifier 24
through a lamp current sensing circuit coupling inductor 30. This
inductor is also a 125 microhenry coil in the preferred embodiment.
A lamp current sensing circuit coupling resistor 32 which has a
resistance of ten ohms connects inductor 30 to ground.
A simmer trigger circuit 38 is also coupled in series to the output
of rectifier 24 through inductor 26 and simmer trigger circuit
coupling resistor 36 for the purpose of initiating triggering of
lamp 28 when the voltage across the lamp exceeds a preset level.
The preferred value for coupling resistor 36 is 301 kilohms. A
transformer primary current sensing circuit 40 is connected to a
coupling resistor 42. The coupling resistor 42 having a resistance
of 0.1 ohms connects first switching means 20 and second switching
means 22 to ground.
One of the essential aspects of the invention is the operation of
the d.c.-to-d.c. converter transformer 12, which generates a higher
output voltage than the maximum required by flashlamp 28 at the
lowest simmer current. Since the voltage of the converter
transformer 12 is higher than the lamp voltage, the current in
coupling inductor 26 increases at a rate determined by the
difference in source and load voltages and the inductance value of
coil 26 and coil 30.
If the simmer current in flashlamp 28 is too low, one of the
switching means 20, 22 will be activated and the voltage at the
output of converter transformer 12 will increase and quickly exceed
the required flashlamp voltage. Full wave rectifying bridge 24
impresses this excess voltage across the twin inductors 26 and 30.
The polarity of this impressed voltage is such that the current
through inductors 26 and 30 will increase with time. Since
flashlamp 28 is connected in series with these coils, the flashlamp
current I(s) increases accordingly. If the current I(s) is too
large or becomes too large, switching means 20, 22 are turned off
and the current through flashlamp 28 is maintained only by the
inductive action of coils 26 and 30. This results in a voltage drop
across inductors 26 and 30 which produces a diminution of flashlamp
current over time.
Lamp current sensing circuit 34 is a conventional logic circuit
which monitors the load current I(s) with the help of inductor 30
and resistor 32. The elements and design of this current sensing
subcircuit are well known to persons ordinarily skilled in the
electronics arts. When I(s) reaches a preselected upper limit, lamp
current sensing circuit 34 turns off converter transformer 12 by
turning off one of the two switching means 20, 22. This change in
output from converter transformer 12 immediately causes the
flashlamp current to diminish at a rate determined by the lamp
voltage and the inductance values of coils 26 and 30. When the
current I(s) falls to its preselected low limit, converter
transformer 12 is activated again and the lamp current I(s)
immediately increases. Each time the d.c.-d.c. converter 12 is
turned on, switching means 20, 22 are toggled so that their
operation alternates. This alternate operation keeps the converter
transformer 12 within its operating range, although both switching
means are turned off during some intervals of operation. If
d.c.-d.c. converter transformer 12 reaches a saturation condition
before the upper limit of lamp current is achieved, the d.c.-d.c.
converter shuts off. It remains in an inactive condition until the
lamp current I(s) reaches the lower limit, and then the opposite
switching means of the converter is turned back on.
When converter transformer 12 is saturated, the increased primary
current is sensed by resistor 42 connected to transformer primary
current sensing circuit 40 and switching means 20, 22. If the two
switching means 20, 22 are field effect transistors, resistor 42 is
connected in series with their common sources. If the primary
current sensing signal exceeds some threshold value of a current
sense comparator 80 (not shown in FIG. 1) within the transformer
primary current sensing circuit 40, a signal is generated which
turns off both switching means 20, 22. The current sense comparator
82 (not shown in FIG. 1) within the lamp current sensing circuit 34
then holds both switching means 20, 22 in an off condition until
the flashlamp current I(s) reaches its lower limit. Since
saturation of converter transformer 12 only occurs when the
switching means 20, 22 have been on for almost the full interval of
time needed to reachs full flashlamp current I(s), the time the two
switching means are held in an off condition will be nearly as
long, as if the deactivation of the switching means 20, 22 had been
caused by reaching the preselected upper limit for I(s). Saturation
normally occurs only for slight imbalances in the winding
resistances of the two halves of the primary winding and
differences in the resistances of the switching means when they are
in an on condition. In the event that the lamp 28 fails in an open
condition, the primary sense circuit 40 will function to toggle the
switching means 20, 22 at a relatively lower frequency as the
transformer 12 is allowed to safely alternate between its two
saturation limits. Under simmering conditions, the frequency of
this switching is, therefore, approximately the same for switching
due to current sensing in the lamp current sensing circuit 34 or in
transformer primary current sensing circuit 40. The switching
frequency is determined by the values of the circuit components and
by the dynamic behavior of flashlamp 28. The switching frequency
may be reduced by increasing the value of inductance of the coils
26 and 30.
The toggling action which alternates the operation of switching
means 20, 22 is controlled by a flip-flop 86 circuit (not shown in
FIG. 1) within transformer primary current sensing circuit 40. As
the two switching means 20 and 22 are alternately activated, the
flux in converter transformer 12 varies back and forth over its
unsaturated range. As the flux cycles, voltage of alternating
polarity is supplied to transformer secondary 16.
The foregoing aspects of the present invention may be seen in more
detail in FIG. 7. Transformer primary current sensing circuit 40 is
comprised of a voltage comparator 80 having a first voltage
reference V.sub.REF1 coupled to a noninverting input thereof and an
inverting input coupled to the top of resistor 42. The magnitude of
V.sub.REF1 is predetermined to be equal to the magnitude of the
voltage appearing across resistor 42 due to the transformer primary
current I.sub.PRIMARY when an overcurrent condition exists in
transformer primary 14. When such an overcurrent condition occurs,
the output of comparator 40 will be low, disabling gates 88 and 90
and turning off switching means 20 and 22, which in FIG. 7 are
shown as FETs 92 and 94. When the transformer primary 14
overcurrent condition is removed, the voltage appearing at the
inverting input of comparator 80 will be less than V.sub.REF1,
causing the output of comparator 80 to go high, reenabling gates 88
and 90 if lamp current sensing circuit 34 is not then sensing an
overcurrent condition of I.sub.s.
Lamp current sensing circuit 34 is similarly comprised of a voltage
comparator 82 which is operable for comparing the voltage across
resister 32, due to I.sub.s, against a second voltage reference
V.sub.REF2. V.sub.REF2 has a magnitude which corresponds to an
I.sub.s overcurrent condition. If comparator 82 detects such an
overcurrent condition, the output will be driven low. The output of
comparator 82 is coupled to the output of comparator 80 such that
when either is low gates 88 and 90 are disabled, thereby turning
off FETs 92 and 94.
The outputs of comparators 80 and 82 are also coupled to the input
of inverter 84 such that when either is driven low the output of
inverter 84 is driven high, thereby clocking, or toggling,
flip-flop 86. Flip-flop 86 has a Q output coupled to gate 90 and a
Q-NOT output coupled to gate 88 such that either FET 94 or FET 92,
respectively, is turned on when no overcurrent condition is
present. The Q-NOT output is further coupled to the D input of
flip-flop 86 such that the Q and Q-NOT outputs are alternately
driven high when flip-flop 86 is clocked by inverter 84.
During the initial period of operation of the simmer power supply
10, the conduction of flashlamp 28 is started by a simmer trigger
circuit 38. This subcircuit simply provides initial ionization of
flashlamp 28, allowing the flow of simmer current which maintains
the flashlamp in a continuously ready condition. The components
within circuit 38 and its design and operation are well known to
persons ordinarily skilled in the electronics arts.
FIG. 2 shows regulation curves 44 and 46 for driving fixed resistor
loads. The regulator voltage V(s) is plotted in curve 44 in volts
versus the resistance of the lamp in ohms. Curve 46 depicts current
through the lamp I(s).
A single cycle of the regulator voltage V(s), current with a
flashlamp load I(s), lamp resistance R(1), and relative diameter of
the flashlamp simmer conducting path d.about. are presented in FIG.
3. A comparison of this data for the present invention with the
performance of previous devices suggests the superb ability of the
Farnsworth circuity to smooth out flashlamp current variations and
restrict them to a much narrower region. Since the present
invention does not lock up and suffer from the synchronization
problems which plague prior devices, the flashlamp in the invention
claimed below operates at a dynamic resistance level and arc
diameter that precludes the snuff-out problem observed in earlier
power supplies.
FIG. 4 illustrates the dynamic range of the simmer power supply.
The single graph plots the flashlamp voltage V(1) against flashlamp
current I(1) across the entire range of operation of the present
invention. The output voltage of transformer 12 is indicated by
horizontal dashed line 60. The intermediate horizontal dashed line
62 represents the maximum lamp voltage at the minimum desired
simmer current. The lowest horizontal dashed line 64 marks the
minimum lamp voltage at the maximum desired simmer current. The
range of simmer current in one preferred embodiment is indicated by
minimum current 66 and maximum current 68. At low current region 56
of the voltage versus current graph, the impedance (R) of the
flashlamp 28 is negative ten kilohms or greater. Low current region
56 is bounded by the lower limit of simmer current range indicated
by dashed vertical line 66. A flashlamp represents a load whose
behavior is nonlinear. When operated at low current its resistance
may become negative. In order to stabilize the operation of the
flashlamp in this negative impedance region, the flashlamp is
driven by a positive impedance larger than the lamp's negative
impedance. High current portion 58 of the voltage versus current
curve is bounded by the upper limit of the range of the simmer
current 68. At this region of operation, the impedance (R) of the
flashlamp is on the order of one or two ohms.
FIG. 5 reveals experimental data obtained during a test of the
present invention. When a pair of field effect transistors are
employed as first and second switching means 20 and 22 in the
primary winding 14 of d.c.-to-d.c. converter transformer 12, the
drain voltage and drain current appear as illustrated in curves 70
and 72 respectively. Voltage curve 70 is measured against a grid
which represents twenty volts per division along the abscissa.
Current curve 72 is measured at five amperes per vertical division.
Both graphs 70 and 72 are plotted against a time scale along the
ordinate that is measured at twenty microseconds per division.
FIG. 6 contains a graph generated during a laboratory test of the
Farnsworth simmer power supply. The flashlamp current is shown by
curve 74 against a scale of twenty milliamperes per division along
the y-axis and a time scale of twenty microseconds per division
along the x-axis.
Because the present invention includes a low value sensing resistor
to sense the flashlamp current, the Farnsworth simmer current
invention further includes a circuit design which is substantially
immune from noise and Joule heating losses which would greatly
debilitate flashlamp performance.
In the preferred embodiment, the strike voltage across the lamp,
measured from cathode to anode, is approximately at least 800 volts
at the moment triggering is initiated. The nominal simmer output
voltage across the lamp takes on some steady state value, usually
between 100 and 200 volts as required by lamp characteristics. The
simmer current is nominally around 60 milliamperes with a maximum
short term deviation cycle of .+-.30 percent. The simmer current is
optimally capable of building up immediately after the flashlamp is
triggered. The lamp voltage is also optimally capable of following
the rapid drop in lamp resistance while the simmer arc is forming.
During steady state operation, the voltage follows short term
variations in lamp resistance caused by simmer supply internal
oscillations. Current regulation is controlled in the secondary of
the transformer. High and low values of current through the series
inductor I(s) are sensed directly and used to open and close the
switching field effect transistors in the primary of the
transformer. One advantage of this embodiment is that there is no
overlap in FET conduction, since they are either both open, when
I(s) is too high and decreasing, or one is closed when I(s) is too
low and increasing. The circuit is arrange to automatically
alternate the FET that is closed when the current needs to be
higher. When one FET is closed and I(s) reaches its preselected
limit, then that FET is opened. After I(s) falls to its minimum
preselected limit, the other FET is closed and the cycle is
repeated as often as is necessary. Another advantage of the
preferred embodiment is that if, during a cycle the transformer
were to saturate due to an uneven match of the switching FET's or
due to an excessively high simmer voltage requirement, a second
sense in the primary winding of the transformer will switch the
activated or conducting FET off and the other half of the winding
will be energized. Yet another important feature of this version of
this innovative invention is that before the simmer current is
initiated, the lamp appears to have an enormous resistance, 3
billion ohms, for example, from the perspective of the simmer
current circuit. At this time before the simmer condition is
imposed, the full voltage of about 800 volts is imposed on this
virtual open circuit. This is accomplished more easily with the
present design because the full secondary is driven by each half of
the primary. Still another advantage which may be realized by
incorporating this important invention into laser radiation devices
is the economy of components required, since the Farnsworth simmer
power supply requires no filter capacitors across the flashlamp.
This improvement leads to much faster responses to variations in
the lamp current as compared to previous devices.
In order for the present simmer power supply circuit to work
properly, the product of input voltage to the transformer at input
tap 18 and the step-up turns ration of the transformer 12 must be
greater than the required flashlamp voltage. Persons ordinarily
skilled in the electronics arts will readily appreciate this design
requirement and would be able to accommodate this operation
condition by providing an appropriate transformer for use with the
Farnsworth invention claimed below.
The Farnsworth simmer power supply continuously maintains the
flashlamp current I(s) between the preselected upper and lower
limits. It provides ideal simmer operation independent of flashlamp
characteristics. This important invention constitutes a major step
forward in the laser and electro-optronic arts.
Although the present invention has been described in detail with
reference to a particular preferred embodiment, persons possessing
ordinary skill in the art to which this invention pertains will
appreciate that various modifications and enhancements may be made
without departing from the spirit and scope of the invention.
* * * * *